Aquifer remediation barrier for removal of inorganic contaminants

ABSTRACT

A permeable barrier for decontaminating groundwater having two distinct components for increasing contaminant removal. Preferably, the barrier material is a mixture or distinct layers of bone-char phosphate material and iron oxide pellets. The barrier material can be incorporated as part of a shallow trenching decontamination system, or incorporated in a non-pumping well, or array of non-pumping wells, as part of a deep underground decontamination system. The system can be used for removing, among other things, a radionuclide, such as uranium, from water.

FIELD OF THE INVENTION

[0001] In general, the present invention relates to the removal ofinorganic contaminants from groundwater. More particularly, thisinvention relates to a sandwich barrier for use with either a passivetreatment system employing an array of non-pumping wells or in a passivetreatment trenching system for removal of such contaminants.

BACKGROUND OF THE INVENTION

[0002] As interest in environmental issues has increased, so hasinterest in improved methods of cleaning the land, air and water. Inthis regard, the interest in improved methods of groundwater cleanup hasbeen acute. Potable ground-water supplies worldwide are contaminated by,or threatened by, advancing plumes containing radionuclides and metals.Unlike air, land and above groundwater cleanups, removal of contaminantsfrom groundwater areas presents special problems with respect to gainingaccess to the contaminated areas which can make these cleanups verydifficult.

[0003] Once a contaminant enters a groundwater system, it can betransported by the groundwater to a great depth. Additionally, suchactivities as mining or deep underground storage of radioactive or toxicmaterials can create a flow of contaminants through groundwater hundredsof feet below the surface. Contamination by uranium and otherradioactive materials is particularly troubling because of the longevityof these materials.

[0004] Conventional methods for treating deep groundwater contaminationare largely unsatisfactory because of cost and other considerations.Active methods that seek out the undergroundwater and bring it to thesurface for treatment are most commonly used. The active methods mostfrequently used are so-called pump and treat methods, trenching methods,or some mixture of the two.

[0005] In a pumping method, contaminated groundwater is pumped from thecontaminated lower regions to the upper regions where the groundwatermay be treated and returned. The water may be brought up to the surfaceusing drilled holes, trenches, or other means that provide the pumpingequipment access to the water. Pump and treat systems suffer fromseveral problems. The power requirements for a pump and treat operationare very high, and can be prohibitively expensive. This is particularlytrue of pump systems that treat deeper (>100 feet below land surface)groundwater. Further, these systems are costly and difficult tomaintain.

[0006] A trenching system employs a trench dug into the ground down tothe point of contamination. The exposed water is pumped from the trenchor treated on site as the water flows through the trench area. Trenchingsystems work well in some instances of shallow contamination but runinto difficulties when applied to contamination deeper within the groundbecause of the high cost of digging the trench, monitoring costs andissues of ground disturbance and recontouring. Another greatdisadvantages of trenching systems is the high cost of pumping thecontaminated water from the trench to the surface for cleanup.

[0007] Because of the high costs of active systems using pump means,interest has increased in passive systems that can treatundergroundwater without a need for actual removal of the water. The useof arrays of non-pumped wells has been proposed as a method to remediatecontaminated groundwater when the installation of treatment wells is notpossible because of technical or financial constraints, for example,when contaminant streams are >200 feet below the land surface.

[0008] Prior art of interest include U.S. Pat. Nos. 5,512,702 (Methodsfor in-situ immobilization of lead in contaminated soils, wastes, andsediments using solid calcium phosphate materials); U.S. Pat. No.5,514,279 (System for treating contaminated groundwater); and U.S. Pat.No. 5,362,394 (System for treating contaminated groundwater).

[0009] Important limitations of prior art treatment materials includethe following: the materials are effective only with respect to a smallnumber of contaminants, and the treatment materials are difficult toreplace.

[0010] It will be evident from the foregoing that there is a need inthis art of contaminated groundwater treatment for a passive, low costmaterials for groundwater treatment that can treat a wide variety ofunderground contaminants including radioactive contaminants, can treat alarge amount of water per unit, can be incorporated into a passivetreatment system, and offers simple retrieval, replacement, and disposalof the reactive material used. In this regard, an alternative, costeffective approach to pump-and-treat methods could be widespreadapplicability to the treatment of contaminated groundwater associatedwith abandoned and active mine sites throughout the United States andother parts of the world.

SUMMARY OF THE INVENTION

[0011] In accordance with the invention, an improved permeable barrieris provided for decontaminating groundwater. The barrier can efficientlytreat a wide variety of contaminants as part of a low-cost passivetreatment system, and the barrier is simple and inexpensive to replace.

[0012] The barrier is disposed in a flow path for groundwater to bedecontaminated and is comprised of a bone-char phosphate and iron oxidepellets.

[0013] In one embodiment, the phosphate and pellets of the barrier are asubstantially uniform admixture.

[0014] In an alternative embodiment, the barrier comprises verticallyalternating layers of the phosphate and pellets.

[0015] Preferably, the barrier is used for decontaminating aradionuclide from water. The radionulide decontaminated from water ispreferably uranium.

[0016] In another aspect of the invention, a system is provided fordecontaminating groundwater, the system comprising an array ofnon-pumping treatment wells disposed in a flow area for groundwater tobe decontaminated, each of the wells containing a barrier comprised ofbone-char phosphate and iron oxide pellets for decontaminatinggroundwater flowing therethrough.

[0017] In yet another aspect of the invention, a system is provided fordecontaminating groundwater, the system comprising a trench disposed ina flow area for groundwater to be decontaminated and a barrier comprisedof bone-char phosphate and iron oxide pellets disposed within the trenchfor decontaminating groundwater flowing therethrough.

[0018] In still another aspect of the invention, a groundwaterdecontamination system is provided, the system comprising a non-pumpingwell decontamination unit, a barrier comprised of bone-char phosphateand iron oxide pellets incorporated within the decontamination unit, andmeans for lowering the unit down through a non-pumping well into anacquifer containing contaminated water.

[0019] Further features and advantages of the present invention will beset forth in, or will be apparent from, the detailed description of apreferred thereof which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a side elevational view of the reactive barrier beingdeployed as part of a decontamination unit in a non-pumping well watertreatment system.

[0021]FIG. 2 is a top plan view of the reactive barrier incorporatedwithin the decontamination unit.

[0022]FIG. 3 is a perspective view of the barrier incorporated in anarray of non-pumping decontamination wells.

[0023]FIG. 4 is a perspective view of the barrier incorporated in atrenching system.

[0024]FIGS. 5a and 5 b are two graphs showing the change in uraniumconcentrations in water samples taken along the perimeter of barrierdeployment tubes in a non-pumping well treatment system in two differentlocations.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0025] Referring to the drawings and, in particular to, FIG. 1, there isshown a permeable reactive barrier 10 incorporated within a deep aquiferremediation tool (referred to by the acronym DART) for decontaminatingdeep groundwater. The barrier 10 is incorporated in a permanent,semi-permanent, or replaceable unit that contains a zone of reactivematerial that acts as a passive in-situ treatment zone.

[0026] This in-situ treatment zone degrades or immobilizes contaminants,such as radionuclides and other trace elements, as the groundwater flowstherethrough. Operational and maintenance costs are lower because waterflow across the barrier 10 is driven by the natural hydraulic gradientbetween the well and aquifer, and the treatment system does not requireoperational maintenance. Reactions within the barrier either degradecontaminants to non-toxic forms or transfer the contaminants to animmobile phase.

[0027] In accordance with the invention, the barrier 10 contains abarrier material comprising a mixture of iron pellets and bone charphosphate. Such a mixture increases the efficiency of removingcontaminants such as uranium from groundwater. The removal of uranium bythis mixture of iron and bone-char phosphate occurs because thephosphate that dissolves from the bone-char pellets is strongly adsorbedby the iron oxides in the mixture. The adsorbed phosphate on the ironsource reacts with uranium in the groundwater to form a low solubilitymetal phosphate, hence removing the uranium from the groundwater. Thebarrier of the invention increases the treatment capacity and allows forlonger treatment periods before material replacement. This results in amore cost effective groundwater treatment program.

[0028] In one embodiment of the invention, the overall tool or systemincludes a plurality of non-pumping wells, one of which is indicated at22 in FIG. 1. In FIG. 3, three non-pumping wells 22 are locateddownstream of contaminant groundwater 18 which flows, as indicated bythe “flow” arrow, into an acquifer 20. Referring to FIG. 1, adecontamination unit 12 is shown as being disposed in aquifer 20 havingbeen lowered down by a cable 15 through a non-pumping well 22 into theaquifer 20 through which the contaminated groundwater flows. As waterflows past the lowered unit 12, the water is passively treated bycontact by the barrier 10.

[0029] Turning to FIG. 2, the decontamination unit 12 has a porous outertube 16 with openings, and a porous inner tube 14 filled with barriermaterial forming barrier 10. As contaminated water 18 flows by the unit12, a portion of the water moves into openings in the outer tube 16 tothe interior of the unit 14. The water 18 comes into contact with aporous inner tube 14 that also has a plurality of openings. The water 18moves through the openings of the inner tube 14 into the interior of theinner tube 14 and the barrier 10 therein. The water entering the innertube 14 comes into contact with the barrier 10 which removescontaminants from inflowing water on contact. Since the barriermaterials within the treatment tools can be deployed through wells,passive treatment of deeper contaminant streams (deeper than 100 feet)that could not be treated with trenching techniques is now possible.

[0030] Referring to FIG. 4, the contaminant barrier of bone-charphosphate and iron oxide pellets can be used in other contaminantremoval systems such as the trench deployment illustrated schematicallyin FIG. 4. The barrier 10 is located at the bottom of a trench 24 in theflow path.

[0031] The barrier can also work as part of a greater decontaminationsystem to clean a large underground area such as a trench system or anarray of non pumping wells. An array of non-pumping wells 22 are drilledat a fixed distance from each other, or trenches are dug where needed toeffect decontamination.

EXAMPLES

[0032] The barrier was tested using three barriers at an abandoneduranium upgrader. The shallow groundwater in the colluvial aquifer iscontaminated with elevated concentrations of uranium that can exceed20,000 micrograms per liter (μg/L). Two different iron sources (naturalred sand and manufactured iron oxide pellets) mixed with bone charphosphate were utilized. Initial results indicate that iron oxidepellets are a superior iron source.

[0033] Installation and Operation

[0034] As shown in FIG. 3, an array of barrier deployment tubes 12containing different proportions of bone char phosphate and foamed ironoxide pellets 10 were placed in 6-inch diameter wells 22 using a cabletool drilling rig (not shown). Use of arrays of non-pumping wells hasbeen proposed as a method to remediate contaminated groundwater when theinstallation of treatment walls is not possible because of technical orfinancial constraints.

[0035] Under natural flow conditions at the site, groundwater convergesto the non-pumping array of wells 22 and the associated barrierdeployment tubes 12 in response to the difference in hydraulicconductivity between the well 22 and aquifer 20. Numerical simulationsof groundwater movement through the non-pumping well array indicate thateach well intercepts groundwater in a portion of the upgradient aquiferapproximately twice the inside diameter of the well.

[0036] Different proportions of bone char phosphate and iron oxidepellets were used to facilitate increased uranium removal fromgroundwater. The iron oxide pellets strongly adsorb the phosphatereleased form the phosphate pellets. The adsorbed phosphate can thenreact with the uranium in the groundwater to facilitate formation ofinsoluble uranyl phosphate compounds. The mechanism of uranium removalis a function of the type of barrier material. The PO₄ barrier materialof pelletized bone charcoal used as a phosphate source facilitatesformation of insoluble uranyl phosphate compounds.

[0037] The following proportions of bone char phosphate:iron oxidepellets (volume ratio) were used in testing, the results of which areshown in FIGS. 5a and 5 b: (1) 50:50 (intermixed); (2) 25:75(intermixed); and (3) 50:50 (layered vertically). The layers of sample(3) are denoted 10 a and 10 b in FIG. 3. Each barrier package had fivemonitoring points for the collection of water samples.

[0038] Results

[0039] Percent uranium removal was calculated using the followingformula:

U _(removed)=100(U _(ban) /U _(input))  (1)

[0040] Where

[0041] U_(removed) is the percent of uranium

[0042] U_(ban) is the concentration of uranium in groundwater 1.5 feetfrom the poa gravel/barrier interface

[0043] U_(input) is the concentration of uranium in groundwater prior toentering the barrier.

[0044] Three months of uranium-concentration data were collected fromthree barrier deployment tubes that were installed in the non-pumpingwell array The results are as shown in FIGS. 5a and 5 b. During thefirst three months of operation, the barrier material removed 95% of theuranium. Excellent results were also obtained using the barrier as partof a trench system (shown in FIG. 4). During the first year of operationthe barrier removed an average of 94 percent of the input uranium fromthe groundwater after traveling 1.5 feet into the trench barrier.

[0045] Although the invention has been described above in relation to apreferred embodiment thereof, it will be understood by those skilled inthe art that variations and modifications can be effected withoutdeparting from the scope and spirit of the invention.

1. A permeable barrier for decontaminating groundwater, said barrierbeing disposed in a flow path for groundwater to be decontaminated andsaid barrier comprising a bone-char phosphate and iron oxide pellets. 2.A permeable barrier for decontaminating groundwater according to claim1, wherein said phosphate and said pellets are a substantially uniformadmixture.
 3. A permeable barrier for decontaminating groundwateraccording to claim 1, wherein said phosphate and said pellets are invertically alternating layers.
 4. A permeable barrier according to claim1, wherein the barrier is used for decontaminating a radionuclide fromwater.
 5. A permeable barrier according to claim 4, wherein theradionuclide is uranium.
 6. A system for decontaminating groundwatercomprising an array of non-pumping treatment wells disposed in a flowarea for groundwater to be decontaminated, each of said wells containinga barrier comprised of bone-char phosphate and iron oxide pellets fordecontaminating groundwater flowing therethrough.
 7. A system fordecontaminating groundwater according to claim 6, wherein said phosphateand said pellets are a substantially uniform admixture.
 8. A system fordecontaminating groundwater according to claim 6, wherein said phosphateand said pellets are in vertically alternating layers.
 9. A system fordecontaminating groundwater, comprising a trench disposed in a flow areafor groundwater to be decontaminated and a barrier comprised ofbone-char phosphate and iron oxide pellets disposed within said trenchfor decontaminating groundwater flowing therethrough.
 10. A permeablebarrier for decontaminating groundwater according to claim 9, whereinsaid phosphate and said pellets are a substantially uniform admixture.11. A permeable barrier for decontaminating groundwater according toclaim 9, wherein said phosphate and said pellets are in verticallyalternating layers.
 12. A groundwater decontamination system comp risinga non-pumping well decontamination unit, a barrier comprised ofbone-char phosphate and iron oxide pellets incorporated within saiddecontamination unit, and means for lowering said unit down through anon-pumping well into an acquifer containing contaminated water.
 13. Apermeable barrier for decontaminating groundwater according to claim 12,wherein said phosphate and said pellets are a substantially uniformadmixture.
 14. A permeable barrier for decontaminating groundwateraccording to claim 12, wherein said phosphate and said pellets are invertically alternating layers.